Polymer-supported reagents are a crosslinked polymer with a functional group capable of triggering a chemical reaction as bonded to the polymer chains. A crosslinked polystyrene is widely used for such polymer supported reagent since a functional group may be more easily introduced thereto. For example, there have been known in the art the polymer supported reagents such as ion exchange resins having a benzene ring with an anionic or cationic group attached thereto or a crosslinked polystyrene with a protective group for a protein synthesis bonded thereto, also well-known as a polymer supported reagent for Merrifield synthesis. Besides, different types of polystyrene supported reagents find their applications in synthesis of various organic compounds and in purification reactions.
The polymer supported reagents are advantageous in that they are easy to separate after a reaction, and can be reused, as well. Moreover, they hardly have toxicity or odor in comparison with a low molecular weight reagent. However, they are very expensive and show a low level of reactivity since the functional group introduced to the polymer chain has a poor mobility. Recently, however, vigorous researches on polymer supported reagents of a novel structure are under way in order to address aforementioned problems. Depending on their intended use, the polymer supported reagents may be utilized as ion exchange resins, substrates for synthesis of proteins or other specific organic compounds, catalysts, agents for separating and transferring a special reagent, and the like. Among them, studies have been made most intensively on the use for the organic synthesis substrates and catalysts.
It has been known that aromatic nitro compounds such as nitrobenzene derivatives may be easily reduced to an aniline compound by means of various hydrogenation catalysts. Metallic reducing agents such as Ranny Ni, PtO2, Zn/HCl, Al/NH4Cl/MeOH, or NaBH4/BiCl3 have been commonly known as the catalyst for such reduction reaction. These catalysts are, however, very expensive, entails using an organic solvent, are sensitive to moisture, and sometimes require a reaction to be carried out at a high pressure.
Catalysts such as Zn/NH4Cl or an ionic liquid have been recently developed in an effort to remedy these shortcomings. By using these catalysts, aromatic nitro compounds such as nitrobenzene may be reduced in water to produce aniline compounds in high yield up to 80%. However, such a reduction reaction alone may not lead to the production of aromatic azoxy-, azo-, or hydrazo-compounds such as expensive azoxybenzene, azobenzene, or hydrazobenzene, which are widely used for a dye, a pigment, an analytic reagent, a reducing agent, a stabilizer, an intermediate for a medicine and in an organic synthesis, or a polymerization inhibitor.
It was previously known that these compounds may be obtained by subjecting an aniline compound as a starting material to an oxidation in the presence of an expensive special catalyst such as sodium tungstate/hydrogen peroxide, magnesium sulfate/aluminum oxide or Au/TiO2 and then subjecting the intermediate product thus obtained, e.g., phenyl hydroxy amine and nitroso benzene to a coupling reaction therebetween. In this method, as a starting material, the aromatic nitro compound such as nitrobenzene is reduced to provide an aniline compound, which is then used for synthesizing an azoxy compound, an azo compound, or a hydrazo compound via the aforementioned coupling reaction. However, the reaction using such expensive catalyst and the multi-stage reduction reaction make the production process extremely complex and complicated and their yield is not really high, as well. Moreover, a mixture of different compounds is disadvantageously produced therefrom.
Therefore, there has been a need for a method of easily producing azoxy-, azo- or hydrazo compounds from the aromatic nitro compound through a more simple reaction process.